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Figuring neutral point on biplane

Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

Figuring neutral point on biplane

Old 11-19-2005, 06:15 PM
da Rock
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Default Figuring neutral point on biplane

I noticed a link to a decent little application that figures a bunch of the a/c design formulas for you. One of the things that saved me a bunch of time is it's ability to figure your stability margin for you. It saves a bunch of time working all that out on paper.

I've used it for a couple of ARFs that seemed to have "bad" balance points specified in their builder's manuals. And now I'm sticking together an Ultimate and figured to check the application's solution for the balance point, but I'm stumbling over the biplane problem. Can't think of a way to describe "the wing" to the application that'd fake it into doing the math for one wing (which is what it wants) yet give a good solution for the two that're actually there.

I guess I need to dig out the Aerodynamics books, but really don't want to. Everytime I disturb those book cases the dust cloud covers half the house.
Old 11-19-2005, 09:54 PM
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Default RE: Figuring neutral point on biplane

Software that supposedly computes a neutral point based on only areas and distances is NOT reliable and should not be used for that purpose. You need aspect ratios, sweep angles, downwash angles, propeller interference and probably a few other things I forgot. However, finding the aero center for a biplane is kinda important. It is the average of the two wings, so use the total area and if there is stagger, split the difference (assuming the wings are the same). If one wing is larger than the other, the aero center will be proportionately closer to the larger wing.

Always start with the CG at 25% MAC of the wing (when the tail is behind the wing) and move back from there based on flight tests. No amount of computing will give you the neutral point exactly and in the case of only using areas and distances as the input, won't even come close!

Why 25%? This is not a ballpark figure. It is an exact figure that represents the lift due to angle of attack (the vast majority). An airfoil that has some curvature may not be exactly at 25% when the angle of attack is 0, but will move towards 25% as the angle of attack increases.

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